1. Field of the Invention
The present invention relates to a process for producing a flame- and smoke-retarded, non-shrinkable, flexible polyurethane foam.
2. Description of the Prior Art
Polyurethane foams are widely used as sheet and cushioning materials for furnitures, vehicles, aircrafts and ships and materials for cloths and buildings due to the characteristic properties. However, the use of polyurethane foams having flame- and smoke-resistance and low toxicity has recently been demanded for these uses. Moreover, the legal regulation of the use of flame- and smoke-retarded polyurethane foam becomes more strict year after year, and this tendency is noticeable in the polyurethane foams for motorcars, railway vehicles and aircrafts. Heretofore, various attempts have been made with respect to the method for providing the flame-resistance to polyurethane foams. As conventional methods, there is known the use of addition type flame retardants, such as phosphorus-containing compound, halogen- and phosphorus-containing compound, antimony oxide and other metal oxides, or the use of reaction type flame retardants. It is, also, well known to be effective as a means of providing the flame-resistance to the polyurethane foams to introduce a ring structure having a heat resistance, such as isocyanurate ring structure or imide ring structure, into the main chain of the polyurethane molecule. However, in the former case, smoke is generated significantly upon burning, and in the latter case, the foaming stability and general physical properties of the resulting foam are undesirable.
The inventors have already found a method for producing a flame- and smoke-retarded, flexible polyurethane foam by a hot cure system process, wherein a mixture of a polyol and a low molecular weight polyhydroxyl compound is reacted with a polyisocyanate in the absence of flame retardants. The resulting polyurethane foam is judged as self-extinguishing by the burning test (ASTM D-1692-59T). Furthermore, the inventors have found a process for producing a flame- and smoke-retarded flexible polyurethane foam judged as non-burning by the burning test, which comprises reacting a mixture of a polyol and a low molecular weight polyhydroxyl compound with a polyisocyanate in the presence of such a little amount of halogenated phosphoric acid ester that cannot give flame resistance to flexible polyurethane foam in the conventional hot cure system process. The resulting polyurethane foam has a great practical value for industrial uses and also is inexpensive owing to the small amount of the flame retardant. However, the resulting foam using a low molecular weight polyhydroxyl compound has a serious drawback in the shrinkage after the foaming due to the formation of a low proportion of closed cells, and is not a satisfactory foam. Especially, in the process of mass production of slab stock, it is an important problem to solve such shrinkage after foaming, and the means to solve such problem has been earnestly required. In order to prevent the shrinkage of the foam originated by the closed cell, two processes are well known, such as a mechanical method (so called "crushing") and a method wherein a cell opening agent (so called "cell-opener") is compounded to the raw materials. However, the former requires large scale facilities and proper selection of reaction condition, and the latter is apt to be accompanied by heterogeneity of cells at the foaming, and is poor in the foaming stability and in the properties of the resulting foam such as flame- and smoke-resistance. That is, effective processes for producing flexible polyurethane foam having both flame- and smoke-resistance and non-shrinkable property are not, heretofore, known.